How to Complete The Whole Truth
The objective is to defeat all foes. The foes are six Cassies. They frequently inflict Charm, so equip Ribbons on units whenever possible. You can also throw a Knot of Rust at a Charmed unit to remove the Charm status without doing too much damage. The Cassies are weak to Wind, so try using attacks like Air Render, and take a look at your weapons to see if any of them are Wind-elemental.